Monday, December 14, 2009
As I've been doing a bit of travel for work of late, I've been boning-up on aircraft and such. One question I don't hear people ask much of is "how high do jetliners fly?" This is known as the service ceiling, and most fly between 25000' and 35000'. The newest planes (the A380, B787, A350, and B747-8) max out at 43000' & the Concorde flew up to 60000'. So how high should we fly?
The U2 operates at up to 85000', but its obviously small and supported in lift by large glider wings. The operation of the A380 at 41000-43000' was granted by an FAA waiver on a 38000' ceiling by suggesting one pilot be wearing an oxygen mask in the event an engine blew apart and hit the plane: that same FAA waiver also suggested reduced CO2 emissions from lower fuel requirements (less dense air = less fuel needed to fly). One thing you also have to do as you go higher is to fly fast: none of these planes mentioned, save the U2, operate with less than Mach 0.85 speed at 35000-38000'. Flight, is all about avoiding a free-fall back to Earth.
Back to the part about the A380 needing a mask for the pilot: a few weeks ago I read this, which goes into some detail about exposure to vacuum. You have 15 seconds top to get decent air in your lungs when you're above 45000', else you'll pass-out from hypoxia and die within minutes. And apparently, the current masks that pop out during air leaks don't really work above 40000': the FAA waiver on the A380 asks for more study on this. Standard procedure for any plane: get down to around 25000' (below Mt Everest height) ASAP.
Last thing to address before planes can start cruising higher: radiation. Concorde had a radiometer for measuring excess radiation from cosmic rays or solar storms: get below 47000' if the count was too high. NOAA has charted exposure levels and links to some sites regarding exposure: apparently frequent fliers and crews can get dosed within the safety limits of other radiation-sensitive work, and prenatal women are advised to consult a doctor about their exposure. There's also a large European document regarding this situation I haven't had a chance to look at yet.